This invention concerns a substrate cleaning method and device, and in particular, a substrate cleaning method and device which is suitable for cleaning a semiconductor substrate (wafer), and in particular a silicon substrate, when an oxide film is formed.
In silicon integrated circuits formed by modern techniques, and in particular in MOS (Metal Oxide Semiconductor) integrated circuits, an extremely thin oxide film is used as a gate insulating film. In sub-micron MOS devices with a gate length of 1.0 .mu.m or less, the film thickness used may for example be 100 .ANG. or less. The objective of reducing the film thickness is to realize improved gain.
With oxide films, for example those used as gate insulating films in MOS devices such as those described above, improvement of electrical characteristics depends very much on ensuring that carbon or other contaminants are not contained in the oxide film when it is formed. To obtain a clean, high quality oxide film without contaminants, pre-oxidation cleaning is widely used as disclosed in, for example, Reference 1: "VLSI Technology" (edited by S. M. Sze, translated into Japanese by Yoshiyuki Takeishi et al, and published by Soken Shuppan)--p. 156.
In the most widely used substrate pre-oxidation cleaning method, carbon, organic and inorganic substances and heavy metal contaminants were removed from the substrate by using chemical reagents such as a mixture of hydrogen peroxide-water-hydrochloric acid solution (H.sub.2 O.sub.2 O-HC1), after which the substrate was washed with pure water. With H.sub.2 O.sub.2 H.sub.2 O-HC1, for example, soluble metal ion complexes are formed, and readhesion of the contaminants to the substrate is thereby prevented.
To form the oxide film after the substrate has been cleaned, it is placed in a reaction furnace. In the furnace, an oxide film is formed on the substrate surface by thermal oxidation. The substrate is usually heated at a relatively low temperature, e.g., below 800.degree. C. for long periods of 30-60 min.
In the conventional cleaning method described above, however, the following problems arose.
(1) Although contaminant particles in the chemical reagents or pure water used to clean the substrate are removed as far as possible, they cannot in practice by removed completely. Even if these substances are used for cleaning, therefore, particulate contaminants in these reagents or water stick to the substrate. (2) Even if contaminants are removed, the oxygen in the atmosphere combines chemically with the substrate at room temperature to form a natural oxide film on the substrate surface.
If for example a gate insulating film is formed on the substrate when contaminants have not been completely removed and a natural oxide film is present, it is difficult to improve the electrical characteristics of the MOS device.